1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling a braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to wheel brake cylinders, and which is also called as an anti-locking control system. The anti-skid control system includes the system for the rear road wheels and that for front and rear road wheels, i.e., four wheel control system. In the case where the road wheels are prevented from being locked according to the former system, the running stability may be ensured and the stopping distance may be minimized. In the case where the front road wheels are also prevented from being locked according to the latter system, the controllability may be maintained.
In view of the fact that, when a hydraulic braking pressure supplied to each wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before the coefficient of friction relative to the road wheel reaches a maximum, the anti-skid control system controls the wheel cylinder pressure according to the deceleration of the vehicle in order that a slip rate of the road wheel results in around 20%, that is, the maximum coefficient of friction is obtained.
As for the control of the braking force applied to each road wheel according to the above anti-skid control system, so called low-select simultaneous control mode is employed particularly for the right and left rear road wheels. According to the low-select simultaneous control mode, a braking force, which is applied to one of the rear road wheels rotating at the lower speed and having a tendency to be locked first, is applied to both of the right and left rear road wheels simultaneously. Consequently, the running stability of the vehicle may be ensured, even in the case where one of the two rear road wheels is positioned on a road of a high coefficient of friction and the other is positioned on a road of a low coefficient of friction, for example. Thus, the braking force applied to the front road wheels is controlled according to an independent control mode for controlling independently the braking force applied to each front road wheel, while the braking force applied to the rear road wheels is controlled according to the simultaneous control mode for controlling simultaneously the braking force applied to both rear road wheels on the basis of the braking force applied to the road wheel rotating at the lower speed.
In Japanese Patent Publication No. Sho 56-28741, there is disclosed a device for changing control modes in the case where two groups of road wheels are controlled by one same actuator. Namely, a low-speed selection mode for controlling the actuator on the basis of the condition of the road wheel having a tendency to be locked first, and a high-speed selection mode for controlling the actuator on the basis of the condition of the road wheel having a tendency to be locked later, are changed over to each other depending upon the braking condition.
According to the simultaneous control mode for controlling simultaneously the braking force applied to both two rear road wheels, when the vehicle is undergoing a cornering maneuver on the road of the high coefficient of friction, for example, a difference in wheel speed between the road wheel located on the inside of the curve in the vehicle's path (hereinafter referred to as inner road wheel) and the road wheel located on the outside of the curve in the vehicle's path (hereinafter referred to as outer road wheel) is produced in turning of the vehicle as is well known, so that the braking force applied to the inner road wheel at the lower speed is possibly selected according to the low-select mode, notwithstanding that the vehicle is running on the road of the high coefficient of friction. Then, the braking force applied to the outer road wheel might be insufficient, so that the stopping distance might be caused to extend. Further, when the vehicle is running on the road of the low coefficient of friction at low speed, if the braking force applied to the two rear road wheels is controlled simultaneously, a considerable load is applied to the rear road wheels in braking operation, so that a vertical vibration of the vehicle is likely to be caused.